Have you ever thought about infinity? Or read what modern cosmologists have to say about the universe? Back a few centuries, people who thought about such things figured that the universe consisted of the earth which was surround by crystal spheres. Astronomers (actually astrologers) of those days never wrote about what was outside the outermost crystal sphere. Then came along Copernicus, Galileo and Newton and the universe expanded somewhat to the size of the solar system and the fixed stars out there somewhere; nobody knew how far. When astronomers gazed through more powerful telescopes and other sophisticated gear, suddenly the universe expanded to billions of light years in size.
But how far did it go? Did it stretch on forever? In the early part of the twentieth century, Albert Einstein proposed his Theory of Relativity. This and other discoveries changed everything. Cosmologists began to think of the universe as a great expanding ball (or some other shape) that had a definite limit.
Okay, that is the current view of our universe. But if our universe is a great ball (or some other shape) curved in the fourth dimension, what lies outside of it? Recent articles I have read about what modern cosmologists and physicist think about the universe speak of "multiverses." I wonder what they are like.
Here are some thoughts on the matter. Not all of them are mine. According to one viewpoint,. the universe is defined to be everything, including space and time itself. Since the universe includes all space, there can be nothing "outside" the universe.
However, the Big Bang theory changed the view of cosmologists and others who study such things completely. According to the Big Bang theory, the Big Bang was not an explosion in a preexisting three-dimensional space, with matter and light expanding out into empty space from some central point; instead, matter and energy are understood to fill all of space (i.e. "the universe"), and what's expanding is space ("the universe") itself. But, wait a minute, if it's expanding, what is it expanding into?
The Big Bang theory is based on Einstein's theory of general relativity, which explains that gravity of matter/energy causes space-time to curve. The amount of curve depends on the average density of matter/energy throughout the universe, a consequence of this is that the universe as a whole can be curved, with either positive curvature, zero curvature, or negative curvature.
To visualize a closed universe with positive curvature, drop the dimensions by one. Instead of curved three-dimensional space, picture a two-dimensional universe in which two-dimensional space is curved into a sphere, and "expanding space" means that the sphere is blowing up like a balloon while the bits of two dimensional matter on the surface do not change in size. You can see that if you pasted a bunch of bits of paper on a balloon and then blew it up, each bit would see the other bits receding from it. This is what astronomers observe, when they view distant galaxies.
If you made a movie of the balloon blowing up and play the movie backwards, after a while the size of the sphere approaches zero, all the bits of matter throughout the balloon universe get more and more squished together and approach infinite density as the size approaches zero. This is what the big bang is supposed to be. Of course, this analogy forces you to picture the two dimensional surface of the sphere expanding into a third dimension, and our curved three dimensional space is expanding into four dimensional space. The question becomes what else is in the fourth dimensional space? Other balloons (universes)?
Also, if space is curved in the fourth dimension, what happens if you travel in a straight line as far as you could go? Would you return to your starting point? And if the fourth dimension is time, would you return to the point in time when you started your journey?
On the other hand, J. Bruno wrote: "The center of the universe is everywhere, as well as its border but does not exist anywhere." Thus, the universe has a center that exists everywhere and borders that exist everywhere. Think about this. Does this statement violate any physical or philosophical rules or not? If the answer is "not," the universe is and will stay infinite and isotropic.
Not everyone agrees. According to Stephen Hawkins, a black hole transports information to another universe. Another theory hypothesizes that there are many (perhaps an infinite number of) universes outside our own. In fact, one theory is that there is a larger universe of which our universe is only a part, like an atom of something much larger.
There is also the theory of parallel universes in which a new universe is created whenever two or more possible futures occur. That each possibility creates a new universe which is almost identical to the ones parallel to it except in one the change occurred one way, and in the other, it occurred the other way.
Of course, all this brings up the question of what is the universe composed. When we say that space is curved, what is this curved "space" composed of? Nothing? How can nothing be curved? It seems that there are more questions than answers. For more confusing answers and questions, go to any web site on modern cosmology.
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